1. Field of the Disclosure
The present invention relates to a laser pumping unit having a tunnel junction and a high power laser device including the same, and more particularly, to a laser pumping unit and a high power vertical external cavity surface 15 emitting laser (VECSEL) of easily diffusing current in a traverse direction by using the tunnel junction including the same.
2. Description of the Related Art
FIG. 1 is a sectional view of a conventional vertical cavity surface emitting laser (VCSEL) 10, which emits a laser beam in a perpendicular direction to a substrate. Referring to FIG. 1, the conventional VCSEL 10 is formed by sequentially stacking a substrate 11 formed of n-GaAs, a lower distributed brag reflector (DBR) layer 13, an active layer 14, and an upper DBR layer 16. The lower and upper DBR layers 13 and 16 operate as reflection layers, and have high reflectance for the oscillation wavelength of a laser. The lower DBR layer 13 is an n-type DBR layer doped with an n-type impurity, and the upper DBR layer 16 is a p-type DBR layer doped with a p-type impurity. A metal contact 17 for supplying a current to the active layer 14 is formed on the upper DBR layer 16. When the current is supplied to the active layer 14, holes and electrons are recombinated in the active layer 14 to generate a light. The light is repeatedly reflected between the upper DBR layer 16 and the lower DBR layer 13 and amplified in the active layer 14. Then, the light is emitted through the upper DBR layer 16, as a laser beam.
In such a VCSEL 10, resistance in a traverse direction is substantially greater than resistance in a vertical direction, and thus the current condenses at the edges of an aperture of the active layer 14. This is referred to as current crowding. Curves A in FIG. 1 illustrate current density profile. Referring to FIG. 1, the current density is lower at the center of the active layer 14 than at the edges of the active layer 14 due to the high resistance in the traverse direction. Accordingly, a single traverse mode oscillation is impossible in the VCSEL 10. In order to prevent such a problem, an oxide layer 15 is formed by oxidizing the edges of the active layer 14 to restrict the size of the aperture which acts as a current injection area, to about 5 pm. An ion injection layer may be used instead of the oxide layer 15. Due to a small aperture of the conventional VCSEL 10, the power of the conventional VCSEL 10 is as small as a few mW.
A vertical external cavity surface emitting laser (VECSEL) is a high power laser. A VECSEL obtains high power (a minimum of hundreds mW) by increasing a gain area by using an external mirror. FIG. 2 is a sectional view of a conventional VECSEL 20. Referring to FIG. 2, the VECSEL 20 includes a laser pumping unit 25 having a substrate 21, a lower DBR layer 22, an active layer 23, and an upper DBR layer 24, and a concave external mirror 26. Laser cavities are formed between the lower DBR layer 22 and the upper DBR layer 24, and between the lower DBR layer 22 and the external mirror 26, respectively. A light generated in the active layer 23 is repeatedly reflected between the lower DBR layer 22 and the upper DBR layer 24 and between the lower DBR layer 22 and the external mirror 26 to reciprocate in the active layer 23. Accordingly, a portion of the light having a predetermined wavelength λ2 is output to the outside as a laser beam through the external mirror 26, and the other portion of the light is reflected to be used in an optical pumping operation.
Methods of exciting the active layer 23 in the VECSEL 20 include an optical pumping method and an electric pumping method. In the optical pumping method, a light beam having a shorter wavelength λ1 than the wavelength λ2 of the laser beam 30 is input to the laser pumping unit 25 through a pump laser 27. In the electric pumping method, a current is supplied to the active layer 23 through a metal contact formed in the upper DBR layer 24, as shown in FIG. 1. However, the electric pumping method in the VECSEL cannot solve the problem of the VCSEL. Furthermore, the VECSEL has a large aperture of about 20 to 100 μm. Accordingly, a problem of current concentrating at the edges of the aperture becomes serious, and it becomes difficult to produce single traverse mode oscillation.
A high power laser 30 shown in FIG. 3 is disclosed in U.S. Pat. No. 6,243,407 applied by Aram Moorairan et al. on Jul. 7, 1997 under the title of “high power laser devices, in order to solve such a problem. Referring to FIG. 3, the laser 30 includes a p-type DBR layer 31, an active layer 32, an n-type DBR layer 33, a substrate 34, and an external mirror 38. In addition, a circular contact layer 35 is formed under the p-type DBR layer 31, and ring-shaped contact layers 36 are formed on the substrate 34 to supply current to the active layer 32 via the contact layers 35 and 36. The substrate 34 has a thickness of about 500 μm and is formed of transparent n-GaAs to transmit an oscillation wavelength. Laser cavities are formed between the p-type DBR layer 31 and the n-type DBR layer 33, and between the p-type DBR layer 31 and the external mirror 38. In addition, a second harmonic generation (SHG) crystal 37 for doubling the frequency of a light may be arranged between the external cavity 38 and the substrate 34.
Referring to FIG. 3, the laser 30 is designed for the beam generated in the active layer 32 to transmit the substrate 34. In other words, the substrate 34 is arranged in the laser cavity, between the p-type DBR layer 31 and the external mirror 38. Accordingly, a current 39 may be sufficiently diffused in a traverse direction while flowing through the relatively thick substrate 34, thereby preventing current crowding. As a result, the laser device 30 may generate a single traverse mode oscillation.
However, the laser 30 has problems. Free carrier absorption due to n-GaAs, which is generally used for the substrate 34, restricts the output and the efficiency of the laser device 30. More specifically, such a problem may be serious because the substrate 34 has a large thickness.
In addition, in a conventional VECSEL, about 30% of the optical energy, which resonates in laser cavities, exists in the laser cavity formed between the upper DBR layer and the lower DBR layer and the other 70% of the optical energy exists in the laser cavity formed between the DBR layer and an external mirror. In the case of the laser device 30 of FIG. 3, about 30% of the optical energy is distributed in the laser cavity formed between the DBR layer 31 and the external mirror 38 in order to reduce the effect of the free carrier absorption. On the other hand, the efficiency of the SHG crystal 37, which is formed between the substrate 34 and the external mirror 38, increases proportionally to the optical energy. As a result, the efficiency of the laser device 30 of FIG. 3 is deteriorated.
Furthermore, the distance of an optical path between the external mirror 38 and the DBR layer 31 is relatively large, and thus a convex surface of the external mirror 38 should be precisely manufactured to precisely converge a beam reflected from the external mirror 38 onto the DBR 31. Accordingly, it is difficult to arrange the SHG crystal 37 at an optimum location.